Public policy concerns are growing, especially in the United States, regarding indoor air quality and its impact on human health. Currently, ventilation air (also called “make-up air”) is needed to maintain acceptable concentrations of indoor-generated air pollutants. Generally, ventilation air must be conditioned prior to use, such conditioning to include subjection to particle filtration, heating or cooling, and humidification or dehumidification as determined by daily weather and seasonal climate conditions. An estimated 20 to 40 percent of energy consumption in buildings goes into conditioning indoor air, much of it directed to treatment of the ventilation air.
Along similar lines, public policy and environmental concerns continue with respect to outdoor air quality and atmospheric pollution. Waste gas streams from industrial processes generate pollutants in quantities that often do not meet air pollution regulations. Accordingly, national laws require treating industrial waste gases to reduce concentrations of pollutants prior to disposing waste gases into the atmosphere; and such laws might become more stringent in the future.
Prior art in this area of technology discloses sorbents for pollutants from indoor and industrial sources, such sorbents including activated carbon, expanded graphite, zeolites and molecular sieves. A majority of gaseous pollutants including polar volatile organic compounds, indoor formaldehyde being one notable example, are weakly adsorbed by activated carbons. Consequently, polar VOC's are removed via chemical reaction with compounds added to the activated carbon, or more typically, are not removed but diluted with ventilation air to acceptable concentrations. Furthermore, where the sorbent must remove several pollutants from the same environmental gaseous source, activated carbon and zeolites suffer from competitive adsorption wherein easily adsorbed compounds replace and outgas weakly adsorbed compounds. Activated carbons and zeolites are also prone to loss of sorbent capacity and mass through oxidation and attrition during thermal regeneration. Regeneration of activated carbons also poses a fire hazard.
Zeolite sorbents are considerably more expensive than activated carbon, but have the advantage of being environmentally benign, non-flammable and thermally stable. Zeolite sorbents are favored for treatment of waste gases and for processes requiring frequent thermal regeneration. A zeolite, however, cannot provide any adsorption area for molecules larger than its pore size or molecules for which it does not have affinity, because a zeolite's adsorption properties towards specific molecules depend heavily on its silicon to aluminum (Si/Al) ratio, cation type, pore structure and acidity. Furthermore, zeolites typically cannot achieve high rates of adsorption for polar pollutants, such as formaldehyde, and preferentially retain water inside their pores limiting sorption of polar VOC's, such that under a relative humidity typical of indoor air in a range of about 40-55 percent, the retention capacity of zeolite sorbents is significantly reduced.
Carbon nanotubes (CNT's) are known to function as sorbents for removing organic compounds from air, as disclosed in international patent application publication no. WO2012/070886 (Bioneer Corporation); or from a liquid or gas as disclosed in US application publication 2003/0024884 (Petrik); or from an exhaust stream as disclosed in U.S. Pat. No. 6,511,527 (Yang, et al.). CNT's are expensive and difficult to produce in large scale as needed for commercialization. US application publication 2006/0191835 discloses a hydrophobic, non-porous, and carbonaceous nanostructured material, specifically graphene, as a sorbent for contaminants including substituted hydrocarbons, organic solvents, and acids. Such publications are silent with respect to regenerability of the sorbent, except for WO2012/070886 which discloses regenerability of the sorbent by heating at a temperature equal to or higher than a catalytic incineration temperature.
The art would benefit from discovery of an improved sorbent for use in a process of removing one or more volatile organic compounds from a gaseous environment, such as air or industrial waste gas streams. Such a sorbent should desirably provide an improved sorbent capacity, especially towards polar VOC's, as well as good thermal and attrition resistance, and improved regeneration capabilities at lower temperatures, as compared with conventional present day sorbents.